A disk drive is a device for storing computer data which can be retrieved at another time. Generally, it comprises a rapidly revolving disk mounted beneath a movable recording head. In a recording mode, data is transferred onto the disk by the recording head as the rotating disk surface moves under the head. The disk surface is coated with a special material which reproduces and stores the data signals coming from the head. Typically, the head can move incrementally over the surface of the disk in a radial direction. By advancing radially in this manner, it is able to lay down concentric tracks of recorded data on the surface of the disk, thereby using an appreciable portion of the disk surface for data storage. In a reading mode, the head merely senses the previously recorded data stored on the disk as the disk surface passes under the head. Just as when the head records the tracks of data, it also accesses the different tracks by advancing incrementally across the disk.
Information is stored on the surface of the disk as bits of data, that is, as 1's and 0's. Because of the nature of the storage medium, however, normal binary data is often stored in accordance with a conversion code. In the case of a magnetic storage medium, for example, one such code is referred to as a 3-for-2 code. That is, normal binary data is divided into two-bit groupings of data, each of which is then represented by a three bit code. The circuitry which prepares the normal binary data for recording onto the disk surface and the circuitry which processes the retrieved data for use elsewhere in the computer system possess the capability of respectively, coding and decoding the data according to the 3-for-2 code.
Generally, the retrieved data is initially handled in the form in which it is retrieved from the disk, i.e. as a serial stream of bits. Thus, the decoding circuitry and the circuitry which assembles the normal binary data into bytes for use elsewhere in the computer system receives its input in serial form.
Along with advances in technology, both the rotating speed of the disk and the stored data densities have increased dramatically. In other words, the rate at which data can be retrieved from the disk has also increased dramatically, requiring correspondingly higher clock rates. As a consequence, the circuitry which subsequently processes the retrieved data must have considerably higher bandwidths to handle the increased data rates. This requirement drives up both the cost and the complexity of the required circuitry.